The invention herein is directed to an improved light scanning assembly for use in the manufacture of printing plates, particularly for lithographic use, by high speed electrostatic methods. The plates produced by the apparatus and method of the invention are superior to printing plates made by known methods whether photographically or electrostatically.
Lithographic printing plates made by the conventional photographic methods are expensive and complex; plates made by electrostatic methods which are known have never been widely used because they require considerable time to produce, have very little life and are low in resolution and spectral response. The latter two disadvantages are characteristic also of photographically made lithographic plates.
Lithographic printing is a process which is basically very old and and well-known. For many years, even well into this century, the technique was practiced on special stone surfaces. A greasy image was applied to a surface, the non-imaged portions being rendered hydrophilic (water attractive, oil repellent). The imaged parts being hydrophobic (water repellent, oil attractive) when a paper receptor is pressed against the surface which has been wetted with water and the greasy ink, the greasy ink having adhered only to the image will be transferred to the paper.
In this process, since the only difference between the imaged and non-imaged areas is the presence of ink on the imaged areas, there is substantially no difference in height between the two areas, this type printing also being known as planographic. In the case of the classic method of letter press printing the imaged areas are in relief above the non-imaged areas or intaglio, that is engraved below the non-imaged areas.
Offset lithography is probably the most important method of printing today. The principle is that ink is offset first from the plate to a rubber blanket and then from the blanket to the paper receptor. There may be an intervening metal drum instead of a rubber blanket. When the printing plate is made, the printing image is rendered hydrophobic, i.e., repellent to water but also attractive to grease. The non-printing areas are rendered just the opposite, that is, hydrophilic. On the press the plate is mounted on a plate cylinder which, as it rotates, comes into contact successively with rollers wet by a water or dampening solution and rollers wet by grease-based ink. The dampening solution wets the non-printing areas of the plate and prevents the ink from wetting these areas. The ink wets the image areas which are transferred to the intermediate blanket cylinder. The paper picks up the image as it passes between the blanket cylinder and the impression cylinder.
In order to appreciate and understand the nature and advantages of this invention, one should comprehend the problems which are a necessary adjunct to the manufacture of a lithographic plate by conventional methods.
Offset plates of conventional construction of the type expected to make many thousands of impressions are expensive to manufacture. Ink receptivity is accomplished by using inherently oleophilic (having an affinity for oil) resins or metals like copper or brass on the image areas. Water receptivity of the non-image areas is usually achieved by using hydrophilic metals like chromium, aluminum or stainless steel and this receptivity is maintained in platemaking and storage by using natural and synthetic gums such as for example, gum arabic.
All offset printing plates which are used for long runs exceeding several thousands of impressions are made by indirect imaging methods. The copy or intelligence is first required to be photographed onto silver halide film and the film negative then used to transfer the image to the printing plate. The transfer is accomplished in all such cases by means of photographic projection onto a coating which is light sensitive and carried by the plate. The negative is used to project the image onto the plate and the processes which follow for the development of the image on the plate vary. Thus, the plates are required to be stored in darkness until used or the light-sensitive coating applied just before use. This is true of the three types of long-run offset plates which are most popularly used today.
The three types of long run plates which are known at this time are surface, deep etch and bimetal. The surface plates are those in which a light-sensitive coating is exposed to a negative, developed etc. The process of achieving the plate requires many steps and treatments. On deep etch plates, after exposure to the negative, the coating in the image areas is removed and coppered chemically lacquered and inked so they are ink receptive. The plate is usually aluminum and the process is quite involved and requires considerable skill. Bimetal plates are similar to deep etch in that the light sensitive coating is removed from the image areas but these areas consist of copper or brass.
By reason of the planographic nature of lithographic plates, electrostatic techniques would seem to lend themselves to the making of such plates. The toned images on a receptor or a photoconductive surface would seem to form the basis for hydrophobic and hydrophilic areas, respectively, but until the invention of the plates of the copending applications, Ser. Nos. 887,189, now U.S. Pat. No. 4,363,367 and Ser. No. 286,609, this has not been realized. Prior attempts, as for example using a zinc oxide electrophotographic member, have not been successful.
Among the difficulties encountered have been low sensitivity, low resolution, mediocre quality, slow speed of manufacture, inability to stand up to wear on a printing press and limited chromatic response. Even conventional lithographic plates are normally exposed only with ultraviolet light and have limited chromatic response. Text and graphics must be separately produced on the plate by complex methods.
Given a metal or polyester based lithographic printing plate wherein the photoconductive coating is as disclosed in U.S. Pat. No. 4,025,339, most, if not all of the disadvantages of prior lithographic printing plates are overcome. The apparatus of the referenced patent application provided solution to an additional group of problems. These included required uniform charging over large area, exposure over the same areas which is capable of accomplishment in a reliable manner by one who is not necessarily skilled in electrostatic techniques, toning and fixing done routinely, with dispatch. The disclosed apparatus of the herein application and the referenced application, Ser. No. 10,497, now U.S. Pat. No. 4,266,869 handles copy originals (both transparencies and opaque originals) conveniently and effects the functional processes so as to enable achievement of the many attributes of the aforementioned printing plates. The apparatus provided in said referenced application Ser. No. 10,497 was easily fabricated, occupied little space, and produced uniform results at each cycle of its operation.
The apparatus described in said referenced application Ser. No. 10,497 for making lithographic printing plates directly from a copy original comprised a light-tight enclosure whose component parts perform all functions needed to deliver a plate, either fully treated and ready for mounting on a press or in condition to be treated by a single bath of etchant and thereby made ready for the press.
A copyboard was mounted horizontally at the top of a light tight enclosure at convenient location to receive a copy original thereon. The copyboard was translated swingably to a substantially vertical disposition where it was scanned progressively by a fixed lamp assembly from its face and/or its rear if the copy original is transparent.
The scanning assembly was formed of a front reflector, parabolic in configuration and extending fully across the front of the copyboard. A tubular lamp faced the pattern when same was oriented vertically. A rear reflector similar to the front reflector also extended fully across the rear of the copyboard and was provided with its own tubular lamp when same is at its vertical disposition. The reflectors and lamps are assembled mounted between plates or brackets located outside of the support standards provided but quite close thereto. The brackets were coupled to an endless chain driven by a drive motor synchronously operated with the drive motor for the carriage so as to move the light scanning assembly up or down illuminating the copyboard progressively along a band across the front and rear face thereof, depending upon whether one or the other of the lamps are energized.
An optical train was arranged to view the whole field of the copyboard and hence, the subject matter of the original was projected onto a charged electrophotographic member by way of a slit that moves over the sensitive surface of the electrophotographic member in synchronism with the scanning of the original.
The electrophotographic member comprised a flexible article based upon a polyester substrate that is transparent or upon a substrate comprising sheet metal that is not as flexible and opaque, the surface of the electrophotographic member that is exposed to the sweeping light beam comprising in each case a sputtered coating of a wholly inorganic photoconductor that has a crystalline structure and is capable of being rapidly charged, imaged and toned as taught in U.S. Pat. No. 4,025,339. The electrophotographic member of Ser. No. 10,497, now U.S. Pat. No. 4,266,869, was carried on a hinged platen which was located at the top of the enclosure but spaced from the "home" position of the copyboard. The hinging arrangement enabled the user to mount the electrophotographic member horizontally in ambient light and then to rotate the member 180 degrees into an aperture in the enclosure while simultaneously closing the aperture, the photosensitive coating side facing into the enclosure.
The slit was provided in a carriage which carried a charging device, said slit, a toning device and excess toner removal means, all moving together with the carriage across the photosensitive surface of the electrophotographic member in synchronism with the light scanning assembly.
The invention herein relates to improvements in the light scanning assembly which increase the versatility of the plate making apparatus disclosed in said referenced application Ser. No. 10,497, which increases lamp life by enabling improved cooling of the lamps, which provides increased light concentration along a narrow band of the copyboard for improved reproduction of the pattern and reduces light losses by employing focused light beams.
In addition to increasing illumination where opaque originals are scanned, considerable improvement is experienced where the original is transparent and light from the rear or backside is directed through the original in a line always focused in the optical system.